The invention refers to a charge/discharge protection circuit for a rechargeable battery which is protected by a fusible link, where the rechargeable battery comprises a control logic which opens or closes a load switch depending on the magnitude of the battery voltage, the voltage on the charge/discharge terminals of the protection circuit and the charge/discharge current.
Such a protection circuit is known as an integrated circuit (IC) with the designation UCC 3952 of Texas Instruments Incorporated. This circuit monitors, among other things, the charge/discharge circuit voltage of the battery and disconnects from the battery via the load switch the charging device when charging and disconnects the load (e.g., a mobile transmitter) when discharging. In addition, the protection circuit monitors the discharge current via a current sensing resistor and opens the load switch when a limit is exceeded (of, e.g., 3 A).
The electric strength of the known protection circuit and the breakdown voltage of the load switch have to be designed for the highest (reasonably to be expected) applied voltage, which for example occurs when connecting a defective charging device or a charging device which was intended for a battery with a higher than the actual voltage or a higher number of cells.
To achieve in an integrated circuit technology a high electric strength, respectively breakdown voltage, large silicon areas and/or special technologies are necessary. Alternatively, though the control logic can be provided with an electric strength commensurate with the actual battery voltage, the load switch then needs to be implemented as an external component with a correspondingly high breakdown voltage.
The task of this invention is based on the requirement to create a charge/discharge protection circuit of the above described type which in normal operation offers the usual functions, whose electric strength, however, needs to be determined only by the actual maximum battery voltage, and which therefore is economical and, when produced in an integrated circuit technology, requires little real estate on an IC chip.
This task is inventively solved with a protection circuit of the above discussed type by providing the control logic with an over-voltage detector. The over-voltage detector is activated and closes a short-circuit switch when the over-voltage detector reaches a fixed voltage limit which in turn depends on the electric strength of the protection circuit. The closing of the short-circuit switch connects the battery terminals via a fusible link.
In the present context the terms xe2x80x9cbatteryxe2x80x9d and xe2x80x9cbattery voltagexe2x80x9d stand for a rechargeable current source or its potential, in particular also for a voltage source comprising only one cell, e.g., a Lithium-Ion cell. It is well known that such voltage sources are typically provided for mobile telephones and are, therefore, subject to special safety rules. Massive overcharging in particular must be reliably prevented because of the associated danger of explosion and fire hazard. This is achieved as proposed by the present invention with a circuit technology with a significantly lower than required electric strength for the worst case condition. And that is accomplished by closing the short-circuit switch when an appropriate predetermined voltage limit is reached, with the resulting short circuit leading to the guaranteed destruction of the fusible link, thus protecting the battery from a dangerous current over-charge.
The proposed embodiment according to the invention makes it possible therefore to economically realize the protection circuit in standard sub-micro technology having a low break-down voltage. If desired, this allows the load switch to be integrated on the same chip with the other components of the protection circuit, whereas now it is frequently realized as a discrete component.
The over-voltage detector preferably receives as input voltage the voltage via the opened load-current switch (claim 2). The voltage limit, at which the over-voltage detector responds, is defined in this case as the voltage just below the break-through voltage of the load-current switch.
Alternatively, the over-voltage detector can receive as input voltage the difference between the voltage at the charge/discharge terminals and the voltage at the battery contacts (claim 3). The voltage limit is then defined as that highest potential at which at least all functionally important circuit components still perform reliably.
When the voltage limit is exceeded it is preferred that the control logic close the previously open load-current switch followed by the time-delayed closing of the short-circuit switch (claim 4). The reason why the load-current switch is open in the presently considered failure mode is that the protection circuit has determined that the maximally allowable load current for normal operation has been exceeded, and has accordingly opened the load-current switch. By closing the load-current switch when the voltage limit is exceeded, dangerously high potentials are reduced via the load-current switch. However, now an inadmissibly high load current flows. This already can lead to the desired melting of the fusible link if the current is high enough. If the current is not high enough then the short-circuit switch will close after a delay time in the range of milliseconds or maximally of seconds and initiates thereby the destruction of the fusible link.
The control logic, appropriately, receives a first supply voltage from the battery, and at least a second supply voltage from an auxiliary voltage source, such as a charged buffer capacitor, when the battery voltage is too low (claim 5). This assures in the presently contemplated failure mode that the protection circuit is supplied with the necessary supply voltage to function until the fusible link is destroyed.
The over-voltage detector preferably includes a bistable flip-flop (claim 7) so that the closing of the short-circuit switch is initiated even when the predetermined voltage limit is exceeded for only a short time.
In one preferred embodiment of the present invention a resistive means coupled between load current switch 3 and charge/discharge terminal 5, shown in FIG. 1 as resistor 4, acts as a current sensor to determine the magnitude of the charge or discharge current. In another preferred embodiment of the present invention the transmission resistance of the load-current switch 3 may be utilized as the current sensing resistance.
Comparators D1, D2 of the control logic 10 are arranged to recognize a battery-side over- or under-voltage, respectively, and the comparator output signals trigger the opening of load switch 3 in the event of an over- or under-voltage.
With the exception of capacitors, at least all circuit elements of low power losses are integrated on one chip. In addition, but again excluding capacitors, all parts of the circuit can be integrated on the chip, including the load switch 3, the short-circuit switch 20, and the fusible link 2.